Lee Dye: Why Some Glaciers Get Dirty

Aug. 21

By Lee Dye

A woman stormed into the mayor's office a while back to demand why Alaska's capital city of Juneau didn't clean up its premier tourist attraction, the spectacular Mendenhall Glacier on the outskirts of town.

The glacier, fed by a 500-square-mile ice field just over the mountains from Juneau, is littered with dirt, marring the view for tourists like herself, the woman complained.

That old yarn makes the rounds up here every year as locals yak it up over the lack of glacial sophistication among the tourists who flock to this region to gaze on some of nature's grand treasures, including the Mendenhall, which is beating a hasty retreat back into the mountains these days like nearly all Alaskan glaciers.

But the tourist had a point that has puzzled experts for years. Why should some glaciers, which form from pristine snow that falls in the mountains, far away from urban populations, be so dirty. Indeed, some glaciers are so filthy that they look like black smudges across the snow rather than the deep blue ice characteristic of glaciers.

And it's not just that the glaciers pick up a little dirt from the fringes as they move back and forth, carving deep "U"-shaped valleys. Dirt, rocks, and all sorts of debris can be found deep inside the ice, far from the edges, frozen in a place where common sense says it shouldn't be.

Like the lady tourist, scientists have also wondered why that should be the case. Now, they think they've figured it out.

Growing Bottoms Up

Experts from several universities, including Lehigh, Michigan State, Buffalo and Penn State, have been examining glaciers in Alaska and Iceland for years now, trying to figure out how the glaciers pick up and transport silt and other debris. The research is funded by the National Science Foundation and the Cold Regions Research and Engineering Laboratory in Hanover, N.H.

Of course, these scientists aren't all that interested in the esthetics that bugged the tourist in Juneau. They are trying to get a better grasp on how glaciers carve those deep valleys and literally move mountains.

That's one of the most dynamic of all mountain-building processes, and to do their job, glaciers must push around thousands of tons of material as they gradually reshape the land.

But that doesn't explain why some of the dirt winds up in the middle of the ice, and is exposed scores of years later as the ice above it melts under the summer sun.

In an article in the recent international journal, Nature, the scientists reveal evidence that glaciers don't owe all their growth to snowfall. They also grow from the bottom up in a process that geologist Grahame Larson of Michigan State compares to opening a can of soda pop that has been in a freezer a bit too long.

Suddenly released from the pressure of the can, the super-cooled liquid soda forms ice on the rim of the can. A similar process occurs in glaciers, the scientists say, and they have given it a name, "glaciohydraulic supercooling."

Why Blue Ice Is Blue

Edward B. Evenson, professor of earth and environmental sciences at Lehigh, says the process begins when the warmer days of summer cause the surface of the glacier to begin to melt. The warmer water penetrates down through the ice to the bottom of the glacier, where it becomes trapped beneath the immense weight of the ice and becomes "supercooled."